Optimizing Reflowed Solder TIM (sTIMs) Processes for Emerging Heterogeneous Integrated Packages

Reflowed indium metal has for decades been the standard for solder thermal interface materials (solder TIMs or sTIMs) in most high-performance computing (HPC) TIM1 applications. The IEEE Heterogeneous Integration Thermal roadmap states that new thermal interface materials solutions must provide a path to the successful application of increased total-package die areas up to 100cm(2). While GPU architectures are relatively isothermal during usage, CPU hotspots in complex heterogeneously-integrated modules will need to be able to handle heat flux hotspots up to 1000W/cm(2) within the next two years. Indium and its alloys are used as reflowed solder thermal interface materials in both CPU and GPU "die to lid/heat spreader" (TIM1) applications. Their high bulk thermal conductivity and proven long-term reliability suit them well for extreme thermomechanical stresses. Voiding is the most important failure mode and has been studied by x-ray. The effects of surface pretreatment, pressure during reflow, solder flux type/fluxless processing, and preform design parameters, such as alloy type, are also examined. The paper includes data on both vacuum and pressure (autoclave) reflow of sTIMs, which is becoming necessary to meet upcoming requirements for ultralow voiding in some instances. The paper also considers the use of solder TIMs in the context of alternative TIM applications and technologies, such as thermal greases, pads, and phase change materials, as well as liquid metals.